From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752143AbcENLSo (ORCPT ); Sat, 14 May 2016 07:18:44 -0400 Received: from smtp34.i.mail.ru ([94.100.177.94]:50590 "EHLO smtp34.i.mail.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750952AbcENLSl (ORCPT ); Sat, 14 May 2016 07:18:41 -0400 Subject: Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack To: Andy Lutomirski References: <0081b876-033b-4fb7-3daf-d38b2df1fda6@list.ru> <97f8e27a-019c-a5d4-2d2c-c2a9627cf23d@list.ru> Cc: Thomas Gleixner , Pavel Emelyanov , Shuah Khan , X86 ML , Andrew Morton , Linux API , Jason Low , "Eric W. Biederman" , Aleksa Sarai , Josh Triplett , "linux-kernel@vger.kernel.org" , Paul Moore , Sasha Levin , Denys Vlasenko , Al Viro , Borislav Petkov , "Amanieu d'Antras" , Konstantin Khlebnikov , Heinrich Schuchardt , Brian Gerst , Tejun Heo , Linus Torvalds , Andrea Arcangeli , Frederic Weisbecker , Palmer Dabbelt , Vladimir Davydov , Oleg Nesterov , Richard Weinberger , "H. Peter Anvin" , Peter Zijlstra From: Stas Sergeev Message-ID: Date: Sat, 14 May 2016 14:18:14 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-Mras: OK Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 14.05.2016 07:18, Andy Lutomirski пишет: > On May 8, 2016 7:05 PM, "Stas Sergeev" wrote: >> 09.05.2016 04:32, Andy Lutomirski пишет: >> >>> On May 7, 2016 7:38 AM, "Stas Sergeev" wrote: >>>> 03.05.2016 20:31, Andy Lutomirski пишет: >>>> >>>>> If a signal stack is set up with SS_AUTODISARM, then the kernel >>>>> inherently avoids incorrectly resetting the signal stack if signals >>>>> recurse: the signal stack will be reset on the first signal >>>>> delivery. This means that we don't need check the stack pointer >>>>> when delivering signals if SS_AUTODISARM is set. >>>>> >>>>> This will make segmented x86 programs more robust: currently there's >>>>> a hole that could be triggered if ESP/RSP appears to point to the >>>>> signal stack but actually doesn't due to a nonzero SS base. >>>>> >>>>> Signed-off-by: Stas Sergeev >>>>> Cc: Al Viro >>>>> Cc: Aleksa Sarai >>>>> Cc: Amanieu d'Antras >>>>> Cc: Andrea Arcangeli >>>>> Cc: Andrew Morton >>>>> Cc: Andy Lutomirski >>>>> Cc: Borislav Petkov >>>>> Cc: Brian Gerst >>>>> Cc: Denys Vlasenko >>>>> Cc: Eric W. Biederman >>>>> Cc: Frederic Weisbecker >>>>> Cc: H. Peter Anvin >>>>> Cc: Heinrich Schuchardt >>>>> Cc: Jason Low >>>>> Cc: Josh Triplett >>>>> Cc: Konstantin Khlebnikov >>>>> Cc: Linus Torvalds >>>>> Cc: Oleg Nesterov >>>>> Cc: Palmer Dabbelt >>>>> Cc: Paul Moore >>>>> Cc: Pavel Emelyanov >>>>> Cc: Peter Zijlstra >>>>> Cc: Richard Weinberger >>>>> Cc: Sasha Levin >>>>> Cc: Shuah Khan >>>>> Cc: Tejun Heo >>>>> Cc: Thomas Gleixner >>>>> Cc: Vladimir Davydov >>>>> Cc: linux-api@vger.kernel.org >>>>> Cc: linux-kernel@vger.kernel.org >>>>> Signed-off-by: Andy Lutomirski >>>>> --- >>>>> include/linux/sched.h | 12 ++++++++++++ >>>>> 1 file changed, 12 insertions(+) >>>>> >>>>> diff --git a/include/linux/sched.h b/include/linux/sched.h >>>>> index 2950c5cd3005..8f03a93348b9 100644 >>>>> --- a/include/linux/sched.h >>>>> +++ b/include/linux/sched.h >>>>> @@ -2576,6 +2576,18 @@ static inline int kill_cad_pid(int sig, int priv) >>>>> */ >>>>> static inline int on_sig_stack(unsigned long sp) >>>>> { >>>>> + /* >>>>> + * If the signal stack is AUTODISARM then, by construction, we >>>>> + * can't be on the signal stack unless user code deliberately set >>>>> + * SS_AUTODISARM when we were already on the it. >>>> "on the it" -> "on it". >>>> >>>> Anyway, I am a bit puzzled with this patch. >>>> You say "unless user code deliberately set >>>> >>>> SS_AUTODISARM when we were already on the it" >>>> so what happens in case it actually does? >>>> >>> Stack corruption. Don't do that. >> Only after your change, I have to admit. :) >> >> >>>> Without your patch: if user sets up the same sas - no stack switch. >>>> if user sets up different sas - stack switch on nested signal. >>>> >>>> With your patch: stack switch in any case, so if user >>>> set up same sas - stack corruption by nested signal. >>>> >>>> Or am I missing the intention? >>> The intention is to make everything completely explicit. With >>> SS_AUTODISARM, the kernel knows directly whether you're on the signal >>> stack, and there should be no need to look at sp. If you set >>> SS_AUTODISARM and get a signal, the signal stack gets disarmed. If >>> you take a nested signal, it's delivered normally. When you return >>> all the way out, the signal stack is re-armed. >>> >>> For DOSEMU, this means that no 16-bit register state can possibly >>> cause a signal to be delivered wrong, because the register state when >>> a signal is raised won't affect delivery, which seems like a good >>> thing to me. >> Yes, but doesn't affect dosemu1 which doesn't use SS_AUTODISARM. >> So IMHO the SS check should still be added, even if not for dosemu2. >> >> >>> If this behavior would be problematic for you, can you explain why? >> Only theoretically: if someone sets SS_AUTODISARM inside a >> sighandler. Since this doesn't give EPERM, I wouldn't deliberately >> make it a broken scenario (esp if it wasn't before the particular change). >> Ideally it would give EPERM, but we can't, so doesn't matter much. >> I just wanted to warn about the possible regression. > I suppose we could return an error if you are on the sigstack when > setting SS_AUTODISARM, although I was hoping to avoid yet more special > cases. Hmm. How about extending the generic check then? Currently it is roughly: if (on_sig_stack(sp)) return -EPERM; and we could do: if (on_sig_stack(sp) || on_new_sas(new_sas, sp)) return -EPERM; Looks like it will close the potential hole opened by your commit without introducing the special case for SS_AUTODISARM. What do you think?